18 July 2015

Purcell Sill → Dike, Grinnell Glacier Cirque, Montana

Posted by Callan Bentley

The Neoproterozoic Purcell Sill is a stark, obvious black stripe in the strata of Glacier National Park. Here it is emerging from behind “The Salamander” glacier, above Grinnell Glacier Cirque:


Zooming in, you can see the “baked” (bleached) zones above and below this concordant intrusion.


But this time, during my visit to this special place, I noticed a discordant offshoot from the main sill:


See it? Up there at the head of that valley:


Zooming in further, you can see four people standing on the contact metamorphosed zone on the left side of this dike:


I went and searched for it in Google Maps, and found it quite easily emerging from that icy valley;


How this escaped my notice during my previous trips to this site, I’ll never know. I was glad to find something new there, though.